Conventionally, power devices using silicon (Si) semiconductors have been used as devices for power electronics. Devices for power electronics are required to operate at a higher frequency with a larger current. Various studies for research and development have been made to improve the performance of silicon power devices.
However, the performance of the silicon power devices is now approaching the theoretical limit thereof. In addition, power devices are occasionally required to operate in severe environments, for example, at a high temperature or under radiation. Silicon semiconductors are not suitable to use in such severe environments. For these reasons, studies are being made regarding devices using semiconductor materials other than silicon.
Among various semiconductor materials, a silicon carbide (SiC) semiconductor has a large forbidden band width (3.26 eV in the case of type 4H) and is superb in electric conduction control and radiation resistance at high temperature. The silicon carbide semiconductor has a breakdown field which is about one digit higher than that of silicon and so has a high withstand voltage, and also has a saturation drift speed of electrons which is about twice as high as that of silicon and so is controllable at a high frequency with a large power. Owing to these physical properties thereof as a semiconductor material, silicon carbide is anticipated as a semiconductor material for power devices operable at a higher frequency with a larger current.
When developing a semiconductor device using a new semiconductor material, it is necessary to develop a technology for forming a reliable ohmic contact having a low resistance to the semiconductor material. Conventionally, nickel (Ni) has been used as an ohmic electrode material for an n-type silicon carbide semiconductor material, and an eutectic or a stack of silicon (Si) and aluminum (Al), or titanium (Ti), has been used as an ohmic electrode material for a p-type silicon carbide semiconductor material.
However, the characteristics of the ohmic contact to especially the p-type silicon carbide semiconductor material have not been satisfactory. Specifically, when the material for a p-type ohmic electrode is thermally treated at a high temperature of about 1000° C. during the formation of the ohmic electrode, the material for the ohmic electrode is aggregated, which decreases the uniformity or generates a stress due to the aggregation. As a result, crystal distortion or transition occurs in the silicon carbide semiconductor, which causes a problem that the crystallinity is decreased.
When ohmic electrodes in general, not only ohmic electrodes to the p-type silicon carbide semiconductor, are formed, a natural oxide layer is generated at a surface of the silicon carbide semiconductor before a metal layer is deposited. This natural oxide layer has a problem of exerting an adverse effect on the ohmic characteristics when the ohmic electrode and the silicon carbide semiconductor are alloyed with each other.
In light of these problems with the formation of an ohmic electrode to the p-type silicon carbide semiconductor, Patent Documents 1 and 2, for example, disclose using a stacking structure of metal layers of nickel, titanium and aluminum or an alloy of titanium and aluminum.
Patent Document 1: Japanese Patent No. 2509713
Patent Document 2: Japanese Patent No. 2940699